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Full text of "Text Book Of Mechanical Engineering"

IOI2

Appendix VI.

features being the blast box D and the reservoir E for the molten
metal. The former is now applied to all new cupolas on account
of its quiet and steady action, the warmed air passing in at
several openings such as M. The reservoir keeps the metal quite
hot until required, and there is the usual tapping door G, as well
as an air pipe F lined with ganister, and a sight hole H. The
charging door c and the damper rods B B are worked from the
charging platform L, and the gases escape at A A. The blast
pipes j j are governed by valves K K, and there is another sight
hole at N.

Pp. 6 and 747. Blackening.—Plumbago is now very much
used for the facing of moulds, being dusted on like the charcoal
powder, and afterwards sleeked with suitable tools. It must be
used sparingly owing to its refractory nature, and the consequent
tendency to close the sand pores and prevent the escape of gas.
Other substances have also been adopted, but plumbago is the
best because it lies firmly on the mould during casting, while
oak-charcoal dust, though otherwise excellent, has a tendency to
float.

CHAPTER  II.

P. 71. Expansion of Cast Iron from the solid to the
liquid state.

An interesting experiment was made by Sir Thos. Wrightson.
He placed a cast-iron ball of 132 oz. weight on a surface of
molten cast iron. At first the ball sank below the surface, which
shewed it had a greater density than that of the liquid ; but in a
few seconds it had just risen level with the surface, having then
an equal density with the liquid. Continuing to rise, about 1\-
of its volume shewed in ,3 or 4 minutes, and at the end of 6
minutes it had melted away. The top of the ball was connected
with a spring balance which carried a pencil that marked the
curves shewn in Fig. 925 upon a clock-driven drum. Comparing
the pressures on the spring, it appeared that the ball expanded
i % before it attained an equal density with the liquid, and a
r expansion of 6% caused a total increase of 7% in volume.